The present invention relates to a phase-modulating light modulator and a method for ensuring minimum amplitude modulation in phase-modulating light modulators where the phase-modulating light modulator comprises an optically active layer with at least one optically active bulk region and with boundary surfaces on which means for the generation of a stationary orientation of the optically active layer are disposed, where the optically active layer comprises liquid crystals with pre-oriented refractive index ellipsoids whose orientation can be controlled discretely for each pixel with the help of the means for the generation of a stationary orientation of the liquid crystals, where the optically active layer is associated with at least one transparent compensation bulk region which comprises at least one birefringent material with fixed refractive index ellipsoids.
Spatial light modulators (SLM), e.g. as used in holographic applications, are optical elements which reflect or transmit in particular visible light and whose optical volume properties can be temporarily modified. The optical volume properties can be discretely modified for each pixel.
The optical volume properties can be temporarily modified e.g. by applying an electric field. The electric field can be controlled individually for small surface areas, which allows the optical properties to be controlled discretely for each pixel but fine enough for many holographic applications. Advantage is taken of this possibility for example in order to modify, i.e. to modulate, an incident wave front during its passage though the light modulator such that, from the observer's distance, it resembles a wave front which is emitted by a real object. If the light modulator is controlled accordingly, a holographic reconstruction of an object becomes possible without the need for this object to be actually present at the time of its observation.
The functional principle of a light modulator is based on an optically active layer whose optical volume properties depend on at least one externally controllable physical parameter and can be influenced specifically by varying that parameter. These physical parameters may be electric field strengths. However, other physical parameters, e.g. sound pressures have already been used successfully for a specific modification of the optical volume properties of optically active layers.
Transmissive light modulators typically have an entry polariser and an exit polariser, while reflective light modulators can be fitted with a combined entry and exit polariser.
The most common functional principle of a light modulator takes advantage of the layer made of birefringent material which is embedded between electrically controllable boundary surfaces, in particular between glass plates, in the form of liquid crystals (LC) whose orientation can be controlled, where the layer can be addressed discretely for each pixel in the form of volume units, which will be referred to as liquid crystal cells hereinafter. The control affects the refractive index ellipsoid of the liquid crystals in the individual liquid crystal cells. A change in the form or orientation of the refractive index ellipsoid in relation to the direction of the passing light varies both the optical path length of the light in the birefringent layer and its effect on the polarisation of the light which passes through it. The refractive index ellipsoid is thus a macroscopic model which describes the direction dependence of an effective refractive index which is exhibited by a certain volume of a birefringent substance in its interaction with light depending on the angle of incidence of the light. The position and form of the refractive index ellipsoid depends mainly on the orientation and the properties of the liquid crystals embedded in the considered volume. However, it is not necessarily identical to their orientation in individual cases. Nevertheless, the refractive index ellipsoid will be used hereinafter in order to characterise unambiguous conditions which depend on an orientation of the liquid crystals in birefringent volumes. When a wave front passes through a light modulator, it will be modified by way of a discrete amplitude modulation and/or phase modulation for each pixel. Because there are no light modulators which would be able to perform those two types of modulation in a fully independent manner in a certain transmission angle range, the light modulators are designed such that they are at least able to perform one type of modulation as efficiently as possible.
One problem in particular with phase-modulating light modulators is that disturbing side-effects may occur, which become manifest in various ways depending on the transmission angle of the light. One major side-effect is the angle dependence of the transmittance of the light modulator, which is hitherto insufficiently compensated when conventional light modulators are used. This leads to an undesired angle-dependent amplitude modulation of a phase-modulating light modulator.
Various types of light amplitude-modulating light modulators are known and widely used in two-dimensional (2D) display devices. They are therefore already designed to serve a large wavelength range and a large viewing angle range. The wavelength dependence of the transmittance is compensated by way of calibration at different wavelengths (red R, green G, blue B). In order to achieve a given transmittance at R, G or B, different voltages must be supplied to the liquid crystal cell for R, G and B.
If an observer looks at a light modulator at an oblique angle within the viewing angle range, there will be an angle dependence due to the fact that the observer only perceives light which passes through the liquid crystal layer under a different angle and which thus interacts with a different refractive index in the refractive index ellipsoid. The light therefore exhibits a different polarisation state at the exit polariser, and the light modulator exhibits a different, angle-dependent transmittance.
Documents EP 0793133 and U.S. Pat. No. 6,141,075 describe a liquid-crystal-based display device, where compensation films of birefringent uniaxial or biaxial material are disposed on boundary surfaces or glass plates of amplitude-modulating light modulators in order to compensate the angle dependence. The birefringent material is oriented such that its refractive index ellipsoid is complementary with that of the liquid crystal layer. Within a certain angular range, the light thus exhibits an effective refractive index which is independent of the viewing angle. The angle dependence of the refractive index of the liquid crystal layer and that of the compensation film substantially compensate each other.
One problem is that this is only possible for a certain angle of the liquid crystals, and thus only for a certain transmittance. A different transmittance is associated with a different liquid crystal angle, to which the compensation film is not adapted. In order to achieve a great contrast of the light modulator, the light modulator is compensated for a good black condition, i.e. for a condition with zero transmittance.
The angle dependence of an amplitude-modulating light modulator can thus be reduced at least partly by applying a compensation film or multiple compensation films for example onto one glass plate or onto both glass plates or by disposing it immediately next to the LC layer. The compensation film comprises a uniaxial or biaxial birefringent material. The refractive indices and the orientation are adapted to a certain condition of the amplitude-modulating light modulator. They are ideally designed such that the sum of the refractive indices of the liquid crystal and compensation film is always the same, irrespective of the angle of incidence of the light. If for example the refractive index ellipsoid of the liquid crystal has a longish, cigar-like shape, and if its semi-major axis is oriented at a right angle to the glass plates, the refractive index ellipsoid of the compensation film must be as flat as a pancake and oriented parallel to the glass plates, where the surface normal of the light modulator usually represents a symmetry axis or major axis of the refractive index ellipsoids of the compensation film.
An amplitude-modulating light modulator is typically optimised such that it exhibits great contrast in a large angular range. For this, the compensation film is adapted to the orientation of the liquid crystals which corresponds with the black condition. Great attention is therein paid to the fact that for compensation of the angle dependence of an amplitude-modulating light modulator the compensation film is adapted to a certain condition of the light modulator with a certain orientation of the liquid crystals.
One drawback of the above-mentioned method is that is not suitable for a wide range of rapidly changing transmittance values, because it always only allows one certain transmittance value to be compensated.
It is further known to compensate the wavelength dependence of phase-modulating light modulators by way of calibrating them at various wavelengths (R, G, B). However, this calibration does not take into account the angle dependence of the transmittance of a phase-modulating light modulator.
Document Somalingam, S: “Verbesserung der Schaltdynamik nematischer Flüssigkristalle für adaptive optische Anwendungen”, doctoral thesis, Darmstadt University of Technology, March, 2006, describes phase-modulating light modulators in the form of liquid crystal cells. According to the initial orientation of the liquid crystals, the liquid crystal cells used are divided into Freedericksz cells, distorted alignment phase (DAP) cells and twisted nematic (TN) cells. They have in common the ability to modulate the phase of the incident light with the help of electric fields.
In Freedericksz cells, the liquid crystals exhibit a positive dielectric anisotropy, so that they are oriented parallel to the electrodes, which is why the maximum phase lag between the two polarisations is achieved in the no-field case.
In DAP cells, the liquid crystals exhibit a negative dielectric anisotropy, so that they are oriented perpendicular to the electrodes, which is why the maximum phase lag is achieved when the crystals have their maximum deflection.
In TN cells, the liquid crystals are arranged such to be twisted against each other, altogether by 90°, so that the polarisation of linear-polarised incident light is turned over the thickness of the cell. If a field is applied, the twisted arrangement of the liquid crystals will be broken, so that the polarisation twist cannot be maintained.
To illustrate this, FIG. 1 shows a schematic diagram of the phase-modulating light modulator 10 which is based on pixel-related Feedericksz cells, more specifically a detail comprising three pixels 1, 2, 3. The light modulator 10 comprises a birefringent layer 8, which comprises liquid crystals 9, and whose optical properties can be controlled by way of applying an electric field between the electrodes 4, 5, 6 and 7, where the electrodes 4, 5 and 6 are supplied with the modulation voltages UM1, UM2, and UM3, and the electrode 7 is supplied with ground potential. The condition of the optical properties which is attained by way of controlling the electric field can be described with the help of refractive index ellipsoids, which are characterised by a ratio of axes and an orientation of their main axes—a major axis and two minor axes perpendicular to the former. The birefringent layer 8 is limited by parallel boundary surfaces 17, 18 on which the electrodes 4, 5, 6 and 7 can be disposed. The electrodes 4, 5, 6 and 7 are disposed at least in the immediate vicinity of the boundary surfaces 17, 18 of the birefringent layer 8, in order to be able to control the liquid crystals 9 discretely for each pixel at a selectivity which is as great as possible. According to FIG. 1, the electric field is controlled discretely for each pixel with the help of electrodes 4, 5 and 6, which are structured so to form pixels on the boundary surface 17 of the birefringent layer 8, by way of applying pixel-specific modulation voltages UM1 to UM3 against a common potential supplied to the electrode 7, which is disposed on the other boundary surface 18 of the birefringent layer 8. The common potential is shown as the common ground potential G. The modulation voltages UM1 to UM3 have different values, which in combination with the common ground potential G at the electrode 7 cause different electric field strengths. The different electric field strengths lead to a different orientation of the liquid crystals 9, namely the orientations 91, 92 and 93 of the liquid crystal molecules in the birefringent layer 8, which results in different positions of the refractive index ellipsoids in the individual bulk regions 11, 12 and 13 of the birefringent layer 8, said regions being exposed to different electric field strengths, and which can be illustrated by a different orientation in relation to the major axes of the refractive index ellipsoids.
FIGS. 2a, 2b and 2c show cross-sectional views of the pixels 1, 2 and 3 of the phase-modulating light modulator 10. To maintain a certain clarity, only the liquid crystals 9, 91, 92, 93 and the lower glass plate 19 and the upper glass plate 20 are shown.
The pixels 1, 2 and 3 comprise birefringent liquid crystals 9, 91, 92 and 93 without twist, i.e. without helical structure. Apart from the marginal regions 14, 15 next to the upper and lower glass plates 19 and 20, respectively, where the liquid crystals 9 are oriented in line with the glass plates 19, 20, the liquid crystals 91, 92 and 93 in the bulk regions 11, 12 and 13 are substantially oriented in parallel to each other. The term ‘parallel orientation’ shall be understood as an arrangement which homogenises the optical properties of the birefringent layer 8 at least in a way which leads to the effect that the optical properties of bulk regions 11, 12 and 13 with dimensions smaller than the pixel size can be described with the help of refractive index ellipsoids which have the same ratio of axes and which are oriented in parallel. To keep things simple, only the term ‘orientation of the liquid crystals 9, 91, 92, 93’ will be used below.
In order to achieve a phase modulation, an electric field changes the polar angle α between the liquid crystals 91, 92, 93 and the glass plate 19, 20 and thus the effective refractive index of the birefringent layer 8. As a consequence, the optical path length through the birefringent layer 8 is changed for the light of a certain polarisation which passes through the birefringent layer 8. This leads to the effect that the light which exits the differently controlled pixels 1, 2 and 3 can exhibit different phase conditions.
The orientations of the liquid crystals 9, 91, 92 and 93, and those of the refractive index ellipsoids at different electric fields between the electrodes (not shown), which are disposed above and below the layer 8, are indicated in FIGS. 2a to 2c. The liquid crystals 9, 91, 92 and 93 can be represented by the refractive index ellipsoids which are shown in the Figure. In the direction of the major axis 61 (z axis of the ellipsoid), the extraordinary refractive index ne applies, while in the perpendicular direction of the minor axes 62 (x, y axes of the ellipsoid) in a uniaxial liquid crystal, the ordinary refractive index n0 applies. In a biaxial liquid crystal with two different minor axes, two values, nx and ny, which are related to the two x, y axes 62, are used, instead of an ordinary refractive index n0. In the case of a uniaxial liquid crystal with ne>n0, the refractive index ellipsoid has the same orientation as the liquid crystal 9, 91, 92, 93.
FIG. 2a shows the pixel 1 without an electric field being applied (UM1=G). The liquid crystals 9, 91 are oriented at a polar angle α1 of 90° to the surface normal 16 of the glass plates 19, 20, i.e. parallel to the upper glass plate 19 and to the lower glass plate 20.
FIG. 2b shows pixel 2 with maximum electric field, where the liquid crystals 92 are oriented at a polar angle α2 of about 0°, with the exception of the marginal regions 14, 15, where boundary surface effects at the glass plates 19, 20 cause the liquid crystals 9 to be oriented substantially parallel to the glass plates 19, 20 irrespective of the electric field strength. However, the marginal regions 14, 15 with the liquid crystals 9 which are oriented in parallel along the glass plates 19, 20 are very thin, so that they can be neglected for the moment when discussing the optical properties of the light modulator 10.
FIG. 2c shows the pixel 3, which is exposed to a medium electric field, where the liquid crystals 93 are oriented at an oblique angle to the glass plates 19, 20, more specifically at a polar angle α3 of about 45°.
The arrows in FIGS. 2a to 2c illustrate the effect when an observer sees light which is transmitted through the light modulator 10 at an oblique angle. The arrow marked S represents light which is transmitted at a right angle, and the arrows marked L and R represent light which is transmitted at an oblique angle from the left-hand side and from the right-hand side, respectively. Because the light passes through the light modulator 10 at different angles, and thus with different orientations in relation to the refractive index ellipsoid, the light is subject to different delay and variations in the polarisation state.
If the light is transmitted at an oblique angle L, R, this usually has the effect that light which is linear-polarised after having passed an entry polariser when it enters the light modulator 10 is no longer linear-polarised when it exits the same. If an exit polariser is used, this non-linear polarisation state is expressed in an amplitude modulation, which is disturbing in the phase-modulating light modulator 10.
FIG. 2c shows the pixel 3, where a medium electric field is applied and where the liquid crystals 93 are oriented at an oblique angle. The variation in the polarisation state and thus the extent of amplitude modulation will be greatest in this pixel when the observer perceives the light which is transmitted in the transmission angle range or viewing angle range L-S-R. The relative orientation of the light in relation to the liquid crystals 93 is changed more drastically compared with the orientations of the liquid crystals 91, 92 shown in FIGS. 2a and 2b. 
The orientation of the liquid crystals 91, 92 and 93 and that of the refractive index ellipsoids indicated in FIGS. 2a to 2c, are just examples of possible orientations in the phase-modulating light modulator 10.
The angle dependence of the amplitude modulation of a phase-modulating light modulator must be compensated in particular if the light sources used for illumination are displaced, or if multiple light sources are used at the same time. Displaceable light sources are required in a holographic display device for example if an observer window is to be tracked to a moving observer in the viewing angle range L-S-R. An observer window in this context is a virtual window in the observer plane, through which the observer sees the holographic reconstruction of an object. Under these circumstances, the light passes through the phase-modulating light modulator at different oblique angles, and the polarisation state of the light is only changed by changing the transmission angle in the viewing angle range L-S-R. If a polarisation filter is used to block undesired polarisation states, the change of the polarisation state will result in an additional amplitude modulation, which will in turn lead to a worse and angle-dependent reconstruction quality.
Compensation films and compensation bulk regions for amplitude-modulating light modulators in conjunction with optically active layers are described in the documents    1) De Bougrenet de la Tocnaye et al.: Complex amplitude modulation by use of liquid-crystal spatial light modulators, Appl. Optics 36, No. 8, 1997, pp. 1730,    2) Lueder, Ernst: Liquid crystal displays, Chichester (et al.): Wiley, 2001 (Repr. 2005) (Wiley-SiD series in display technology), ISBN: 0-471-49029-6,    3) US 2004/0155997 A1, and    4) DE 689 17 914 T2.
One problem is that the angle dependence of the transmittance or reflectance and thus of the amplitude modulation cannot be reduced substantially in phase-modulating light modulators in a large viewing angle range L-S-R by using the aforementioned compensation films and compensation bulk regions.